Deletion sequence: the missing amino acid impurity

What is a deletion sequence?

Synthetic peptides are built one amino acid at a time. A deletion sequence is a chain where one of those additions did not happen, so the finished molecule is the target sequence minus one residue (or more) somewhere along its length. Chemists often name them with the prefix des, as in des-Gly for a chain missing a glycine.

How do deletion sequences form?

In solid-phase peptide synthesis, the growing chain is anchored to a resin. Each cycle removes a protecting group from the end of the chain and then couples the next amino acid. If deprotection is incomplete, or a coupling does not go to completion, some chains miss that cycle and carry on growing without the residue. A review of synthetic peptide quality traced deletions to this kind of failure and found impurities with at least one deletion to be especially abundant among those it identified (Verbeke et al., 2015).

The opposite error, where an extra residue is added because excess amino acid was present, produces an insertion sequence. The same review lists deletions and insertions alongside other classes such as aspartimide residues, racemisation and oxidation.

How is a deletion sequence different from a truncated sequence?

In practice the terms overlap and some reports use them loosely. What matters on a certificate is that the impurity is named, or at least separated and counted. See truncated sequence for the related term.

Why are deletion sequences hard to see in HPLC?

Because they look so much like the target. Missing one residue out of many changes the molecule's behaviour on the column only a little, so the impurity can elute just before or after the main peak, or under it. Researchers analysing synthetic glucagon noted that impurities with sequences similar to the main component have retention times close to it and can be masked by the main peak (Wang et al., 2020). When that happens the deletion is counted as product and HPLC purity reads high. See co-elution.

How do laboratories detect and identify them?

By mass. A deletion sequence weighs less than the target by roughly the mass of the missing residue, so LC-MS can pick it out even when UV cannot separate it. The glucagon study identified its peptide impurities this way, noting that each had a different mass-to-charge value from the main component. MS/MS or peptide mapping can then show which residue is missing.

Reference standard programmes use several orthogonal methods, including mass spectrometry, peptide mapping and amino acid analysis, to rule out misincorporation of this kind before a material is accepted as a standard (McCarthy et al., 2023).

What does a deletion sequence mean for a buyer reading a record?

It means the vial contains some molecules that are not quite the labelled peptide. A small, separated deletion peak is a normal part of synthetic peptide impurity profiles and is already reflected in the purity figure. A large unexplained peak near the main one, or a purity figure from a method that cannot resolve close impurities, deserves a closer look.

The lesson Identity first explains why mass matters, and Reading a mass spectrum shows what a lower-mass impurity looks like. The methods are described on LC-MS and Peptide mapping.